A polyethersulfone (PES) electrospun nanofiber mat was evaluated as a membrane for liquid filtration. To alleviate difficult handling of the nanofibrous web and also to provide mechanical strength a poly(ethylene terephthalate) (PET) non-woven was used as the sub-layer. The PES/PET electrospun nanofibrous membranes (ENMs) were characterized in terms of water flux and retention performance. The water flux measurements indicated that the membranes possess a high initial flux. Upon increase of the feed pressure the porosity of the ENFs is deformed resulting in drastically decreased fluxes. To enhance the interfacial stability a heat treatment was performed. Heat treatment approach in addition to enhancement of interfacial stability could preserve the structure of the nanofibrous layer and its effective porosity. Retention tests with polystyrene suspension demonstrated that the filtration performance of the ENMs is highly dependent on size distribution of the suspended particles. When the particles over 1 mu m in size (microparticles) are present in the feed, the major rejection of the particles is performed within the first hour of the measurement. The flux is very high and pressure difference very low and almost negligible. In the case of a feed containing only nanoparticles (<1 mu m in size), the major rejection is accomplished within the first hour. As a consequence of pore blocking, the flux declines and the pressure difference rises drastically in less. This research demonstrates the filtration potential of electrospun nanofibrous membranes for pre-treatment of water and other liquid separations. (C) 2010 Elsevier B.V. All rights reserved.